This invention generally relates to the preparation and use of non-crystalline high molecular weight hydrocarbon soluble drag reducing polymers.
More specifically this invention relates to a method for producing a highly active, non-hazardous, easily transported and easily handled suspension of drag reducing polymers having increased performance characteristics.
It is known that certain polymers which are oil soluble may be polymerized in the presence of catalysts to produce high molecular weight non-crystalline hydrocarbon soluble materials by various means. These polymers, when dissolved in a hydrocarbon fluid flowing through a conduit, greatly reduce turbulent flow and decrease "drag" thus reducing the amount of horsepower needed to move a given volume of fluid or conversely, enabling greater volumes of fluid to be moved with a given amount of power. Further, dilute solutions of high molecular weight polymers in solvents such as hydrocarbons, display useful flow characteristics unusual to the commonly known crystalline, largely non-soluble, artifact-forming polymers such as polyethylene and polypropylene. In particular, these hydrocarbon soluble materials are noted for their effectiveness as drag reducing agents and anti-misting agents. An anti-misting agent is a polymer which, when dissolved in a hydrocarbon, serves to significantly increase medium droplet size and thereby reduce flammability of fuel sprays caused by high velocity wind shear, such as that as which occurs during rupture of a fuel cell resulting from impact such as an airplane crash.
One important aspect of such polymers are their susceptibility to shear degradation when dissolved in hydrocarbons. Thus, passage through a pump, severe constrictions in the pipeline or the like where turbulent flow is encountered can shear the polymer and thereafter reduce its effectiveness. Consequently, it is important that these polymers be placed into the flowing hydrocarbon in a form which achieves certain desirable features.
The polymer should be placed in a form adequate for easy transportation and handling without exotic or unusual equipment, since injection points can often be at remote and inaccessible locations. The polymer must also be a form which dissolves rapidly in the hydrocarbon being transported, since the polymers have little drag reducing effect until solubilized. The polymer should also be innocuous to the ultimate purpose of the hydrocarbon fluid. For example, in the case of a crude oil flowing through a pipeline, larger amounts of water and contaminants can be tolerated than in a finished pipeline product such as diesel fuel or gasoline which are ultimately destined to be consumed in internal combustion engines or the like.
At the present time, a number of different commercial approaches have been taken to the problem of preparing, dissolving, transporting and using such drag reducing polymers. In use, these polymers form extremely dilute solutions (ranging up to about 100 parts per million polymer in hydrocarbon) which are effective in order to achieve drag reduction or anti-misting. The common commercial method is the preparation of dilute solutions of polymer in an inert solvent such as kerosene or other solvating material as set forth in Mack, U.S. Pat. No. 4,433,123. Mack utilized a solution of high molecular weight polymer suitable for use as a drag reducing agent when produced by polymerization of alphaolefin in a hydrocarbon solvent. The entire mixture, containing polyolefin, solvent and catalyst particles, is used without separation to form dilute solutions of the polymer in crude oil or other hydrocarbon. However, one disadvantage of such an approach is the use of a solvent, which poses a shipping and handling hazard. In addition, the product itself forms a gel-like solution which requires pressurized injection equipment and which becomes extremely viscous and difficult to handle under cold weather temperature conditions at pipeline injection points.
In addition, using solution polymerization processes, it has been found necessary to terminate reactions at no more than 20% polymer based on total reactor content by weight in order to obtain the high molecular weight polymers in an effective form, as taught in U.S. Pat. Nos. 4,415,714, 4,493,903, and 4,945,142.
A second approach is to prepare the polymers as a solid material as described in the Weitzen patent, U.S. Pat. No. 4,340,076. Weitzen taught that a high molecular weight polymer would very rapidly dissolve in solvents if the polymer was ground very fine at cryogenic temperatures and the resulting polymer particles were introduced into the solvent while below the glass transition point of the polymer. Polymer concentrations as high as 15 percent or more could easily be obtained, although only a few parts per million were needed for drag reduction. Three patents issued to O'Mara et al., U.S. Pat. Nos. 4,720,397, 4,826,728 and 4,837,249 all deal with finely grinding or commuting the polymers in an inert atmosphere below the glass transition point of the polymer in the presence of a partitioning agent to form a multi-layered coating which holds the inert atmosphere adjacent to the polymer particles after being ground. The patentee teaches it is mandatory that the comminution be carried out in an inert atmosphere utilizing a refrigerant such as liquid nitrogen and a coating agent while grinding the polymer to a size of less than about 0.5 millimeters or about 35 mesh. This process requires that particles be maintained in an inert atmosphere without contacting water or oxygen until dissolved in the hydrocarbon solvent.
Thus, it is necessary either to have an impervious particle coating which prevents air or water from contacting the particle until dissolved, or the entire system must be maintained under an inert atmosphere until the material is dissolved. This process would be bulky and cumbersome, especially at remote pipeline locations.
In the most successful commercial approach, the entire reaction mixture of a polymerization process, comprising a high molecular weight polymer dissolved in a polymerization solvent such as hexane, heptane or kerosene in a concentration ranging from as low as 2 to 3 percent to a maximum of about 12 percent is utilized. While this material has proven to be difficult to pump under cold weather conditions, it is presently the most economical way to utilize polymer reaction products as drag reducing materials.
All commercial processes or known processes to date for the preparation of polymers useful for drag reducing or anti-misting purposes have significant disadvantages. Clearly, dissolving solid particles directly into crude oil requires injecting solids into liquids, or as an alternative, pre-dissolving the solids into liquids and injecting liquid into the flowing hydrocarbon. Either method requires additional equipment not normally utilized in pipeline locations and pump stations, and requires significant handling equipment investment. For example, U.S. Pat. No. 4,340,076, requires a source of liquid nitrogen for its operation. The present commercial approach utilizing the entire polymerization mixture containing solvent is unwieldy because the polymer solution must be transported as a hazardous material and stored in pressure vessels. The product is a viscous polymer solution, making handling unwieldy under low temperature conditions. The limitation on polymer content of 10 to 15 percent in current commercial processes also means that a great deal of shipping cost is incurred relative to the active polymer content.